Method of heterogeneous catalyzed vapor-phase oxidation of propane to acrolein and/or acrylic acid

ABSTRACT

In a process for heterogeneously catalyzed gas-phase oxidation, a reaction gas starting mixture comprising propane, molecular oxygen and, if desired, inert gas is passed at from 300 to 500° C. over a fixed-bed catalyst.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the heterogeneouslycatalyzed gas-phase oxidation of propane to acrolein and/or acrylicacid, in which a reaction gas starting mixture comprising propane,molecular oxygen and, if desired, inert gas is passed at from 300 to500° C. over a fixed-bed catalyst.

2. Discussion of the Background

Acrolein and acrylic acid are important intermediates which areemployed, for example, for producing active compounds and polymers.

At present, by far the most widely employed process for the industrialpreparation of acrolein and/or acrylic acid is the gas-phase catalyticoxidation of propene (for example EP-A 575897), with the propene beingmostly produced as a by-product of ethylene production by steam crackingof naphtha. Since the other application areas for propene, e.g. theproduction of polypropylene, continue to expand, it would beadvantageous to have an industrially usable, competitive process forpreparing acrolein and/or acrylic acid which uses as raw material notpropene but propane which, for example, occurs naturally in largeamounts as a constituent of natural gas.

EP-A 117146 proposes preparing acrylic acid from propane by convertingpropane into a propylene-containing product stream by means ofheterogeneous catalytic dehydrogenation in the absence of molecularoxygen in a first stage and, in subsequent oxidation stages, passingthis product stream together with molecular oxygen over suitableoxidation catalysts so as to oxidize the propene present therein toacrolein and/or acrylic acid.

A disadvantage of this procedure is that it necessarily requires aplurality of reaction stages, with the individual reaction stages havingto be carried out under different reaction conditions.

Furthermore, the abovementioned procedure has the disadvantage that thecatalyst required for the nonoxidative dehydrogenation of the propane isrelatively quickly deactivated as a result of carbon deposits and has tobe regenerated. Since the dehydrogenation product mixture also containshydrogen, CN-A 1105352 casts doubt on the possibility of passing thedehydrogenation product mixture on directly to a subsequent oxidationstage.

Both CN-A 1105352 and Y. Moro-oka in Proceedings of the 10thInternational Congress on Catalysis, Jul. 19-24, 1992, Budapest,Hungary, 1993, Elsevier Science Publishers B. V., pp. 1982 to 1986,recommend first converting propane partially into propene in ahomogeneous oxidative dehydrogenation and converting this propene,without separating it off beforehand, into acrolein and/or acrylic acidin subsequent heterogeneously catalyzed oxidation stages. Disadvantagesof this procedure are, on the one hand, that carbon is also formed in ahomogeneous oxidative dehydrogenation of propane to propene and, on theother hand, that the selectivity of the formation of the desired product(acrolein and/or acrylic acid) is not satisfactory in such a procedure.Thus, in the examples in CN-A 1105352, the selectivity of propeneformation achieved by homogeneous oxidative dehydrogenation is only ≦40%by volume and Moro-oka is also restricted to selectivities of acroleinformation of 64 mol %, based on propane reacted.

It has also been proposed that a heterogeneously catalyzed oxidativedehydrogenation of propane (which is not necessarily accompanied bycarbon formation) be coupled with a subsequent heterogeneously catalyzedoxidation of the propene thus produced to give acrolein and/or acrylicacid (e.g. 210th ACS National Meeting, Chicago, Aug. 20-24, 1995 or WO97/36849). However, further details regarding the type and manner of thecoupling (in general, both reaction steps require reaction conditionswhich cannot be reconciled) were not given. CN-A 1105352 even advisesdecidedly against such a coupling, since, at reasonable propaneconversions, achievable selectivities of propene formation in aheterogeneously catalyzed oxidative dehydrogenation do not exceed thosein a homogeneous oxidative dehydrogenation.

Topics in Catalysis 3(1996), pp. 265-275, reports the heterogeneouslycatalyzed oxidative dehydrogenation of propane to propene over cobaltmolybdate and magnesium molybdate. A disadvantage of the procedure ofthe abovementioned reference is that it is, presumably to ensure asatisfactory selectivity of propene formation, carried out in highdilution, i.e. the reaction gas starting mixture comprising propane andmolecular oxygen further comprises up to 75% by volume of molecularnitrogen (inert gas). Such a high proportion of inert gas does notencourage coupling with a subsequent propene oxidation, since it reducesthe space-time yields of acrolein and/or acrylic acid achievable in asingle pass. Furthermore, such a proportion of nitrogen makes it moredifficult to recirculate unreacted propane and/or propene after havingseparated off acrolein and/acrylic acid, should this be intendedsubsequent to the propene oxidation.

Journal of Catalysis 167 (1997), 550-559 likewise reports theheterogeneously catalyzed oxidative dehydrogenation of propane topropene or molybdates. A disadvantage of the procedure in this referenceis that it likewise recommends the use of a reaction gas startingmixture whose proportion of molecular nitrogen is 70% by volume.Furthermore, the abovementioned reference proposes a dehydrogenationtemperature of 560° C. Such a high dehydrogenation temperature likewisedoes not suggest coupling to a downstream heterogeneously catalyzedpropene oxidation, since it damages the multimetal oxide compositionscustomarily used for an oxidative conversion of propene into acroleinand/or acrylic acid.

Journal of Catalysis 167 (1997), 560-569 likewise recommends adehydrogenation temperature of 560° C. for a heterogeneously catalyzedoxidative dehydrogenation. Similarly, DE-A 19530454 similarly recommendstemperatures above 500° C. for a heterogeneously catalyzed oxidativedehydrogenation of propane to give propene.

Furthermore, experiments on a heterogeneously catalyzed direct oxidationof propane to acrolein and/or acrylic acid are reported in theliterature (e.g. Proceedings, 210th ACS National Meeting, Chicago, Aug.20-24, 1995, FR-A 2693384 and 3rd World Congress on Oxidation Catalysis,R. K. Grasselli, S. T. Oyama, A. M. Gaffney and J. E. Lyons (Editors),1997 Elsevier Science B.V., pp. 375-382), although in these studies,too, either the reported selectivity of the acrolein and/or acrylic acidformation and/or the reported yield of acrolein and/or acrylic acid on asingle pass are not satisfactory.

EP-B 608838 likewise relates to the heterogeneously catalyzed directoxidation of propane to acrylic acid. However, a disadvantage of themethod disclosed in EP-B 608838 is that the selectivities of the acrylicacid formation reported by way of example in this document cannot bereproduced. Thus, our attempts to repeat the work gave a vanishingselectivity for acrylic acid formation. Instead, formation of acroleinwas found when repeating these examples, but the selectivity for theacrolein formation was only ≦30 mol %.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for theheterogeneously catalyzed gas-phase oxidation of propane to acroleinand/or acrylic acid, in which a reaction gas starting mixture comprisingpropane, molecular oxygen and, if desired, inert gas is passed at from300 to 500° C. over a fixed-bed catalyst, which process does not havethe disadvantages of the methods described and/or recommended in theprior art.

DETAILED DESCRIPTION OF THE INVENTION

We have found that this object is achieved by a process for theheterogeneously catalyzed gas-phase oxidation of propane to acroleinand/or acrylic acid, in which a reaction gas starting mixture comprisingpropane, molecular oxygen and, if desired, inert gas is passed at from300 to 500° C. over a fixed-bed catalyst which comprises two catalystbeds A and B arranged spatially in succession, with the proviso that theactive composition of bed A is at least one multimetal oxide of theformula I

M_(a) ¹Mo_(1−b)M_(b) ²O_(x)

where M¹=Co, Ni, Mg, Zn, Mn and/or Cu, preferably Co, Ni and/or Mg,particularly preferably Co and/or Ni,

M²=W, V, Te, Nb, P, Cr, Fe, Sb, Ce, Sn and/or La, preferably Sn, W, P,Sb and/or Cr, particularly preferably W, Sn and/or Sb,

a=0.5-1,5, preferably 0.7-1.2, particularly preferably 0.9-1.0,

b=0-0.5, preferably >0-0.5 and particularly preferably 0.01-0.3, and

x=a number which is determined by the valence and amount of the elementsdifferent from oxygen in I,

and the active composition of bed B is at least one multimetal oxide ofthe formula II

Bi_(a), Mo_(b), X¹ _(c), X² _(d), X³ _(e), X⁴ _(f), X⁵ _(g), X⁶ _(h),O_(x),  (II),

where

X¹=W, V and/or Te, preferably W and/or V

X²=alkaline earth metal, Co, Ni, Zn, Mn, Cu, Cd, Sn and/or Hg,preferably Co, Ni, Zn and/or Cu, particularly preferably Co, Ni and/orZn,

X³=Fe, Cr and/or Ce, preferably Fe and/or Cr,

X⁴=P, As, Sb and/or B, preferably P and/or Sb,

X⁵=alkali metal, Tl and/or Sn, preferably K and/or Na,

X⁶=rare earth metal, Ti, Zr, Nb, Ta, Re, Ru, Rh, Ag, Au, Al, Ga, In, Si,Ge, Th and/or U, preferably Si, Zr, Al, Ag, Nb and/or Ti,

a′=0.01-8, preferably 0.3-4 and particularly preferably 0.5-2,

b′=0.1-30, preferably 0.5 to 15 and particularly preferably 10-13,

c′=0-20, preferably 0.1 to 10 and particularly preferably 0.5-3,

d′=0-20, preferably 2-15 and particularly preferably 3-10,

e′=0-20, preferably 0.5-10 and particularly preferably 1-7,

f′=0-6, preferably 0-1,

g′=0-4, preferably 0.01-1,

h′=0-15, preferably 1-15 and

x′=a number which is determined by the valence and amount of theelements different from oxygen in II,

wherein the reaction gas starting mixture comprises ≧50% by volume ofpropane, ≧15% by volume of O₂ and from 0 to 35% by volume of inert gasand flows through the catalyst beds A and B in the order first A, thenB.

Preferred multimetal oxides I are accordingly those of the formula I′

[Co, Ni a./o. Mg]_(a) Mo_(1−b) [Sn, W, P, Sb a./o. Cr]_(b)O_(x)  (I′),

where a=0.5-1.5, preferably 0.7-1.2, particularly preferably 0.9-1.0,

b=0-0.5, preferably >0-0.5 and particularly preferably 0.01-0.3, and

x is a number which is determined by the valence and amount of theelements different from oxygen in I′.

Particularly preferred multimetal oxides I are those of the formula I″

[Co a./o. Ni]_(a) Mo_(1+b) [W,Sn a./o. Sb]_(b)O_(x)  (I″),

where a, b and x are as defined above.

Preferred multimetal oxides II are those of the formula II′

Bi_(a), Mo_(b), W_(c), [Co,Nia./o.Zn]_(d), Fe_(e), [Pa./o.Sb]_(f),[Ka./o.Na]_(g), X⁶ _(h), O_(x),  (II′),

where X⁶ and the stoichiometric coefficients are as defined for formulaII.

Particularly preferred multimetal oxides II′ are those in which X⁶=Si,Zr, Al, Nb, Ag and/or Ti, among which preference is in turn given tothose in which X⁶=Si.

It is also advantageous for e′ to be 0.5-10, particularly when X⁶=Si.

The above applies particularly when the multimetal oxide compositionsII′ as described in EP-B 575897 are being prepared.

Particularly advantageous catalyst bed pairs A, B are the combinationsI′, II′ and I″, II′. This applies particularly when X⁶=Si and e′=0.5-10.

In the process of the present invention, the reaction gas startingmixture is advantageously passed over the fixed-bed catalyst comprisingcatalyst beds A and B at from 325 to 480 or 450° C., preferably from 350to 420° C. and particularly preferably from 350 to 400° C. Normally, thecatalyst beds A, B have identical temperatures.

If the catalyst bed pair A, B used is a combination I′, II′ or I″, II′,the reaction temperature in both beds is advantageously from 350 to 420°C., frequently from 350 to 400° C.

Furthermore, the reaction gas starting mixture advantageously comprises≦30% by volume, preferably ≦20% by volume and particularly preferably≦10% by volume or ≦5% by volume, of inert gas. Of course, the reactiongas starting mixture can also contain no inert gas. In the presentcontext, inert gases are gases which are reacted to an extent of ≦5 mol% on passing through the fixed-bed catalyst to be used according to thepresent invention. Possible inert gases are, for example, H₂O, CO₂, CO,N₂ and/or noble gases.

Moreover, the reaction gas starting mixture advantageously comprises≧60% by volume or ≧70% by volume or ≧80% by volume of propane. Thepropane content of the reaction gas starting mixture to be usedaccording to the present invention is generally ≦85% by volume,frequently ≦83 or ≦82 or ≦81 or ≦80% by volume. The amount of molecularoxygen present in the reaction gas starting mixture can be up to 35% byvolume in the process of the present invention. It is advantageously atleast 20% by volume or at least 25% by volume. Reaction gas startingmixtures which are useful according to the present invention comprise≧65% by volume and ≦85% by volume of propane plus ≧15% by volume and≦35% by volume of molecular oxygen. According to the invention, it isadvantageous (in respect of selectivity and conversion) for the molarratio of propane to molecular oxygen in the reaction gas startingmixture to be <5:1, preferably ≦4.75:1, better ≦4.5:1 and particularlypreferably ≦4:1. As a rule, the abovementioned ratio will be ≧1:1 or≧2:1.

In principle, active compositions I which are suitable according to thepresent invention can be readily prepared by making up an intimate,preferably finely divided dry mixture of suitable sources of theirelemental constituents which has a composition corresponding to theirstoichiometry and calcining this mixture at from 450 to 1000° C. Thecalcination can be carried out either under inert gas or in an oxidizingatmosphere such as air (mixture of inert gas and oxygen) or else in areducing atmosphere (e.g. mixture of inert gas, oxygen and NH₃, COand/or H₂). The calcination time can be from a few minutes to a fewhours and usually decreases with increasing temperature. Suitablesources of the elemental constituents of the active multimetal oxidecompositions I are compounds which are already oxides and/or compoundswhich can be converted into oxides by heating, at least in the presenceof oxygen.

Apart from the oxides, suitable starting compounds are, in particular,halides, nitrates, formates, oxalates, citrates, acetates, carbonates,amine complexes, ammonium salts and/or hydroxides (compounds such asNH₄OH, (NH₄)₂CO3, NH₄NO₃, NH₄CHO₂, CH₃COOH, NH₄CH₃CO₂ and/or ammoniumoxalate, which dissociate and/or can be decomposed at the latest duringthe subsequent calcination to form compounds which are all given off ingaseous form, can be additionally incorporated into the intimate drymixture). The intimate mixing of the starting compounds to producemultimetal oxide compositions I can be carried out in dry or wet form.If it is carried out in dry form, the starting compounds areadvantageously used as finely divided powders and are subjected tocalcination after mixing and, if desired, compaction. However, theintimate mixing is preferably carried out in wet form. Here, thestarting compounds are usually mixed with one another in the form of anaqueous solution and/or suspension. Particularly intimate dry mixturesare obtained in the mixing process described if all the sources of theelemental constituents used are present in dissolved form. The solventused is preferably water. The resulting aqueous composition issubsequently dried, with the drying process preferably being carried outby spray drying of the aqueous mixture at outlet temperatures of from100 to 150° C. Particularly suitable starting compounds of Mo, V, W andNb are their oxo compounds (molybdates, vanadates, tungstates andniobates) or the acids derived from these. This applies particularly tothe corresponding ammonium compounds (ammonium molybdate, ammoniumvanadate, ammonium tungstate).

In the process of the present invention, the multimetal oxidecompositions I can be used either in powder form or after shaping togive particular catalyst geometries; shaping can be carried out beforeor after the subsequent calcination. For example, unsupported catalystscan be produced from the powder form of the active composition or itsuncalcined precursor composition by compaction to give the desiredcatalyst geometry (e.g. by tableting or extrusion). If desired,auxiliaries such as graphite or stearic acid as lubricants and/orshaping aids and reinforcing materials such as microfibers of glass,asbestos, silicon carbide or potassium titanate can be added. Suitableunsupported catalyst geometries are, for example, solid cylinders orhollow cylinders having an external diameter and a length of from 2 to10 mm. In the case of the hollow cylinders, a wall thickness of from 1to 3 mm is advantageous. Of course, the unsupported catalyst can alsohave a spherical geometry, in which case the sphere diameter can be from2 to 10 mm.

Of course, the shaping of the pulverulent active composition or itspulverulent but not yet calcined precursor composition can also beachieved by application to preshaped inert catalyst supports. Thecoating of the support bodies to produce the coated catalysts isgenerally carried out in a suitable rotatable container, as is known,for example, from DE-A 2909671 or EP-A 293859. To coat the supportbodies, it is advantageous to moisten the powder composition to beapplied and to dry it again after application, e.g. by means of hot air.The thickness of the layer of powder composition applied to the supportbody is advantageously selected so as to be in the range from 50 to 500μm, preferably in the range from 150 to 250 μm.

Support materials which can be used here are customary porous ornonporous aluminum oxides, silicon dioxide, thorium dioxide, zirconiumdioxide, silicon carbide or silicates such as magnesium silicate oraluminum silicate. The support bodies can have regular or irregularshapes, with preference being given to support bodies having a regularshape and a distinct surface roughness, e.g. spheres or hollowcylinders. Essentially nonporous, spherical steatite supports which havea rough surface and a diameter of from 1 to 8 mm, preferably from 4 to 5mm, are useful.

As regards the preparation of the multimetal oxide compositions II, whathas been said for the multimetal oxide compositions I applies. However,the calcination temperature employed is generally from 350 to 650° C.Particularly preferred multimetal oxide compositions II are themultimetal oxide compositions I disclosed in EP-B 575897, particularlytheir preferred variants. In these, multimetal oxides are firstpreformed from portions of the elemental constituents and are used aselement source in the further course of the preparation.

The process of the present invention is advantageously carried out inmultitube reactors as are described, for example, in EP-A 700893 andEP-A 700714. The fixed-bed catalyst to be used according to the presentinvention is located in the metal tubes (in general of stainless steel)and a heat transfer medium, in general a salt melt, is passed around themetal tubes. In the simplest case, the two catalyst beds A and B to beused according to the present invention are arranged in directsuccession in each reaction tube. The ratio of the bed volumes of thetwo catalyst beds A and B is, according to the present invention,advantageously from 1:10 to 10:1, preferably from 1:5 to 5:1 andparticularly preferably from 1:2 to 2:1, frequently 1:1. The reactionpressure is generally ≧0.5 bar. Normally, the reaction pressure will notexceed 100 bar, i.e. it will be from ≧0.5 to 100 bar. It is frequentlyadvantageous for the reaction pressure to be from >1 to 50 bar orfrom >1 to bar. The reaction pressure is preferably ≧1.25 or ≧1.5 bar or≧1.75 or ≧2 bar. Frequently, the upper limit of 10 or 20 bar is notexceeded here. Of course, the reaction pressure can also be 1 bar (theabove statements regarding the reaction pressure apply quite generallyto the process of the present invention). Furthermore, the spacevelocity is advantageously selected such that the residence time of thereaction gas mixture over the two catalyst beds A and B is from 0.5 to20 sec, preferably from 1 to 10 sec, particularly preferably from 1 to 4sec and frequently from 3 sec. Propane and/or propene present in theproduct mixture from the process of the present invention can beseparated off and returned to the gas-phase oxidation according to thepresent invention. Furthermore, the process of the present invention canbe followed by a further heterogeneously catalyzed oxidation stage as isknown for the heterogeneously catalyzed gas-phase oxidation of acroleinto acrylic acid, into which the product mixture of the process of thepresent invention, if desired with the addition of further molecularoxygen, is conveyed. At the end of this, unreacted propane, propeneand/or acrolein can again be separated off and returned to the gas-phaseoxidation. The acrolein and/or acrylic acid formed can be separated fromthe product gas mixtures in a manner known per se. In general, thepropane conversion achieved using the process of the present inventionis ≧5 mol %, or ≧7.5 mol %. However, propane conversions of ≧20 mol %are not normally achieved. The process of the present invention isparticularly suitable for continuous operation. If necessary, additionalmolecular oxygen can be. injected at the level of the catalyst bed B.Conversion, selectivity and residence time referred to in this documentare, unless indicated otherwise, defined as follows:$\text{Conversion of propane (mol \%)} = {\frac{\text{Number of moles of propane reacted}}{\text{Number of moles of propane fed in}} \times 100}$$\text{Selectivity S of acrolein and/oracrylic acid formation (mol \%)} = {\frac{\text{Number of moles of propane converted into acrolein and/or acrlic acid}}{\text{Number of moles of propane reacted}} \times 100}$$\text{Residence time (sec)} = {\frac{\text{free volume of the reactor (1) in the zone where the catalyst is located}}{\text{amount of reaction gas starting mixture passed through}} \times 3600}$

EXAMPLES Example 1

a) Preparation of a Multimetal Oxide Composition I

292.4 g of ammonium heptamolybdate (81.5% by weight of MoO₃) weredissolved at 80° C. in 1.2 kg of water and the resulting solution wasadmixed with 742.4 g of aqueous cobalt nitrate solution (12.5% by weightof Co, based on the solution). The solution formed was evaporated whilestirring on a water bath at 100° C. until a paste-like mass had beenformed. This was subsequently dried for 16 hours at 110° C. in a dryingoven and subsequently calcined in a stream of air in a muffle furnace(60 l internal volume, air throughput: 500 l/h), as follows: Thetemperature was first increased from room temperature (25° C.) to 300°C. at a heating rate of 120° C./h. The temperature of 300° C. wassubsequently held for 3 hours and the calcination temperature was thenincreased from 300 to 550° C. at a heating rate of 125° C./h. Thistemperature was subsequently held for 6 hours. The resulting multimetaloxide was comminuted and the particle size fraction having a particlediameter of from 0.6 to 1.2 mm was separated by sieving as thecatalytically active multimetal oxide composition I of the stoichiometryMo₁Co_(0.95)O_(x).

b) Preparation of a Multimetal Oxide Composition II

1. Preparation of a Starting Composition

50 kg of a solution of Bi(NO₃)₃ in aqueous nitric acid (11% by weight ofBi, 6.4% by weight of HNO₃, in each case based on the solution) wasadmixed with 13.7 kg of ammonium paratungstate (89% by weight of WO₃)and stirred for 1 hour at 50° C. The suspension obtained was spray-driedand calcined for 2 hours at 750° C. The resulting preformed calcinedmixed oxide was milled and the particle size fraction 1 μm≦d≦5 μm(d=particle diameter) was separated out. This particle size fraction wassubsequently mixed with 1% of its weight of finely divided SiO₂ (numberaverage particle diameter: 28 nm).

2. Preparation of a Starting Composition 2

48.9 kg of Fe(NO₃)₃ were dissolved in 104.6 kg of cobalt nitratesolution (12.5% by weight of Co, based on the solution). The resultingsolution was added to a solution of 85.5 kg of ammonium heptamolybdate(81.5% by weight of MoO₃) in 240 l of water. The resulting mixture wasadmixed with 7.8 kg of an aqueous mixture containing 20% of its weightof colloidal SiO₂, and with 377 g of an aqueous solution containing 48%by weight of KOH. This mixture was subsequently stirred for 3 hours andthe resulting aqueous suspension was spray-dried to give the startingcomposition 2.

3. Preparation of the Multimetal Oxide II

The starting composition 1 was mixed with the starting composition 2 inthe amount required for a multimetal oxide II of the stoichiometry

Mo₁₂W₂Bi₁Co_(5.5)Fe₃Si_(1.6)K_(0.08)O_(x),

pressed to form hollow cylinders having a length of 3 mm, an externaldiameter of 5 mm and a wall thickness of 1.5 mm and subsequentlycalcined as follows. The calcination was carried out in a stream of airin a muffle furnace (60 l internal volume, 1 l/h of air per gram ofcatalyst precursor composition). The temperature was first increasedfrom room temperature (25° C.) to 190% at a heating rate of 180° C./h.This temperature was held for 1 hour and then increased to 220° C. at aheating rate of 60° C./h. The 220° C. was again held for 2 hours beforebeing increased to 260° C. at a heating rate of 60° C./h. The 260° C.was subsequently held for 1 hour. The mixture was then first cooled toroom temperature, thus essentially concluding the decomposition phase,and then heated to 465° C. at a heating rate of 180° C./h and thiscalcination temperature was held for 4 hours.

200 g of the resulting active composition were comminuted and theparticle size fraction from 0.6 to 1.2 mm was sieved out as activemultimetal oxide composition II.

c) Gas-phase Catalytic Oxidation of Propane

A reaction tube (V2A steel; 2.5 cm wall thickness; 8.5 mm internaldiameter; electrically heated) having a length of 1.4 m was charged,from the bottom upward on a catalyst base (7 cm length), first to alength of 13 cm with quartz granules (number average particle diameterfrom 1 to 2 mm) and subsequently to a length of 42.5 cm with the activemultimetal oxide composition II and then to a length of 42.5 cm with theactive multimetal oxide composition I, before the charge was completedto a length of 13 cm with quartz granules (number average particlediameter from 1 to 2 mm).

The reaction tube charged as described above was heated to 430° C. overits entire length and then supplied, from the top downward, with 56standard l/h of a reaction gas starting mixture consisting of 80% byvolume of propane and 20% by volume of oxygen.

In a single pass, a product gas mixture having the followingcharacteristics was obtained:

Propane conversion: 10 mol %

Selectivity of acrolein formation: 59 mol %

Selectivity of acrylic acid formation: 14 mol %

Selectivity of propene formation: 3 mol %

Example 2

a) Preparation of a Multimetal Oxide Composition I

The preparation of the multimetal oxide composition I was carried out asin Example 1a), but the final calcination temperature was 560° C.instead of 550° C.

b) Preparation of a Multimetal Oxide Composition II

The preparation of the multimetal oxide composition II was carried outas in Example 1b).

c) Gas-phase Catalytic Oxidation of Propane

A reaction tube (V2A steel; 2.5 cm wall thickness; 8.5 mm internaldiameter; electrically heated) having a length of 1.4 m was charged,from the bottom upward on a catalyst base (7 cm length), first to alength of 18 cm with quartz granules (number average particle diameterfrom 1 to 2 mm) and subsequently to a length of 42.5 cm with the activemultimetal oxide composition II and then to a length of 42.5 cm with theactive multimetal oxide composition I, before the charge was completedto a length of 30 cm with quartz granules (number average particlediameter from 1 to 2 mm).

The reaction tube charged as described above was heated to 415° C. overits entire length and then supplied, from the top downward, with 56standard l/h of a reaction gas starting mixture consisting of 80% byvolume of propane and 20% by volume of oxygen. The pressure at the inletof the reaction tube was 1.68 bar (absolute). The pressure drop alongthe reaction tube was 0.27 bar.

In a single pass, a product gas mixture having the followingcharacteristics was obtained:

propane conversion: 11.5 mol %

selectivity of acrolein formation: 66 mol %

selectivity of acrylic acid formation: 10 mol %

selectivity of propane formation: 2 mol %

Example 3

a) Preparation of a Multimetal Oxide Composition I

877.2 g of ammonium heptamolybdate (81.5% by weight of MoO₃) weredissolved at 45° C. in 3.6 kg of water and the resulting solution wasadmixed with 2227.2 g of aqueous cobalt nitrate solution (12.5% byweight of Co based on the solution).

The resulting clear, red solution was spray-dried in a spray dryer fromNiro (A/S Niro Atomizer transportable Minor) at an inlet temperature of330-340° C. and an outlet temperature of 110° C. 450 g of thespray-dried powder obtained were kneaded (1 l sigma-blade kneader fromWerner & Pfleiderer) with 75 ml of H₂O for 40 minutes and dried at 110°C. for 16 hours in a convection drying oven. The dried powder wassubsequently calcined in a rotating (15 revolutions/min) round-bottomedquartz flask through which air flowed (internal volume: 2 l, airthroughput: constant 250 l/h) as follows (tilting oven heating):

The temperature was first raised from room temperature (25° C.) to 225°C. at a heating rate of 180° C./h. The temperature of 225° C. wassubsequently held for 0.5 hour and the calcination temperature as thenincreased from 225° C. to 300° C. at a heating rate of 60° C./h. Thistemperature was subsequently held for 3 hours. The calcinationtemperature was then increased from 300 to 550° C. at a heating rate of125° C./h. This temperature was subsequently held for 6 hours.

The multimetal oxide thus obtained was comminuted and the particle sizefraction having a particle diameter of from 0.6 to 1.2 mm was separatedout by sieving as the catalytically active multimetal oxide compositionI of the stoichiometry Mo₁Co_(0.95)O_(x).

b) Preparation of a Multimetal Oxide Composition II

The preparation of the multimetal oxide composition II was carried outas in Example Ib).

c) Gas-phase Catalytic Oxidation of Propane

A reaction tube (V2A steel; 2.5 cm wall thickness; 8.5 mm internaldiameter; electrically heated) having a length of 1.4 m was charged,from the bottom upward on a catalyst base (7 cm length), first to alength of 18 cm with quartz granules (number average particle diameterfrom 1 to 2 mm) and subsequently to a length of 42.5 cm with the activemultimetal oxide composition II and then to a length of 42.5 cm with theactive multimetal oxide composition I, before the charge was completedto a length of 30 cm with quartz granules (number average particlediameter from 1 to 2 mm).

The reaction tube charged as described above was heated to 390° C. overits entire length and then supplied, from the top downward, with 84standard l/h of a reaction gas starting mixture consisting of 80% byvolume of propane and 20% by volume of oxygen. The pressure at the inletof the reaction tube was 2.7 bar (absolute). The pressure drop along thereaction tube was 0.35 bar.

In a single pass, a product gas mixture having the followingcharacteristics was obtained:

propane conversion: 9.0 mol %

selectivity of acrolein formation: 69 mol %

selectivity of acrylic acid formation: 10 mol %

selectivity of propane formation: 1 mol %

Example 4

a) Preparation of a multimetal oxide composition I

929.3 g of ammonium heptanemolybdate (81.5% by weight of MoO₃) weredissolved at 45° C. in 1.5 kg of water. 80.8 g of ammonium paratungstate(89.04% by weight of WO₃) were dissolved separately at 95-98° C. in 1.5kg of water and subsequently cooled to 45° C. The two solutions werecombined at 45° C. and admixed with an aqueous cobalt nitrate solution(12.5% by weight of Co based on the solution) which was likewise at 45°C.

The resulting clear, red solution was spray-dried in a spray dryer fromNiro (A/S Niro Atomizer transportable Minor) at an inlet temperature of320° C. and an outlet temperature of 110° C. 450 g of the spray-driedpowder obtained were kneaded (1 l sigma-blade kneader from Werner &Pfleiderer) with 85 ml of H₂O for 40 minutes and dried at 110° C. for 16hours in a convection drying oven. The dried composition wassubsequently calcined in a stream of air in a muffle furnace (internalvolume: 60 l, air throughput: constant 500 l/h) as follows.

The temperature was first raised from room temperature (25° C.) to 300°C. at a heating rate of 120° C./h. The temperature of 300° C. wassubsequently held for 3 hours and the calcination temperature was thenincreased from 300° C. to 567° C. at a heating rate of 125° C./h. Thistemperature was subsequently held for 6 hours.

The multimetal oxide obtained in this way was comminuted and theparticle size fraction having a particle diameter of from 0.6 to 1.2 mmwas separated out by sieving as the catalytically active multimetaloxide composition I of the stoichiometryCo_(0.95)Mo_(0.95)W_(0.05)O_(x).

b) Preparation of a Multimetal Oxide Composition II

The preparation of the multimetal oxide composition II was carried outas in Example 1b).

c) Gas-phase Catalytic Oxidation of Propane

A reaction tube (V2A steel; 2.5 cm wall thickness; 8.5 mm internaldiameter; electrically heated) having a length of 1.4 m was charged,from the bottom upward on a catalyst base (7 cm length), first to alength of 18 cm with quartz granules (number average particle diameterfrom 1 to 2 mm) and subsequently to a length of 42.5 cm with the activemultimetal oxide composition II and then to a length of 42.5 cm with theactive multimetal oxide composition I, before the charge was completedto a length of 30 cm with quartz granules (number average particlediameter from 1 to 2 mm).

The reaction tube charged as described above was heated to 395° C. overits entire length and then supplied, from the top downward, with 98standard l/h of a reaction gas starting mixture consisting of 80% byvolume of propane and 20% by volume of oxygen. The pressure at the inletof the reaction tube was 2.69 bar (absolute). The pressure drop alongthe reaction tube was 0.36 bar.

In a single pass, a product gas mixture having the followingcharacteristics was obtained:

propane conversion: 8.2 mol %

selectivity of acrolein formation: 67 mol %

selectivity of acrylic acid formation: 11 mol %

selectivity of propane formation: 1 mol %

We claim:
 1. A process for the heterogeneously catalyzed gas-phaseoxidation of propane to acrolein and/or acrylic acid, in which areaction gas starting mixture comprising propane, molecular oxygen and,if desired, inert gas is passed at from 300 to 500° C. over a fixed-bedcatalyst which comprises two catalyst beds A and B arranged spatially insuccession, with the proviso that the active composition of bed A is atleast one multimetal oxide of the formula I M_(a) ¹Mo_(1−b)M_(b)²O_(x)  (I), where M¹=Co, Ni, Mg, Zn, Mn and/or Cu, M²=W, V, Te, Nb, P,Cr, Fe, Sb, Ce, Sn and/or La, a=0.5-1.5, b=0-0.5 x=a number which isdetermined by the valence and amount of the elements different fromoxygen in I, and the active composition of bed B is at least onemultimetal oxide of the formula II Bi_(a), Mo_(b), X¹ _(c), X² _(d), X³_(e), X⁴ _(f), X⁵ _(g), X⁶ _(h), O_(x),  (II), where X¹=W, V and/or Te,X²=alkaline earth metal, Co, Ni, Zn, Mn, Cu, Cd, Sn and/or Hg, X³=Fe, Crand/or Ce, X⁴=P, As, Sb and/or B, X⁵=alkali metal, Tl and/or Sn, X⁶=rareearth metal, Ti, Zr, Nb, Ta, Re, Ru, Rh, Ag, Au, Al, Ga, In, Si, Ge, Thand/or U, a′=0.01-8, b′=0.1-30, c′=0-20, d′=0-20, e′=0-20, f′=0-6,g′=0-4, h′=0-15, x′=a number which is determined by the valence andamount of the elements different from oxygen in II, wherein the reactiongas starting mixture comprises ≧50% by volume of propane, ≧15% by volumeof O₂ and from 0 to 35% by volume of inert gas and flows through thecatalyst beds A and B in the order first A, then B.
 2. A process asclaimed in claim 1, wherein the temperature is from 325 to 450° C.
 3. Aprocess as claimed in claim 1, wherein the temperature is from 350 to420° C.
 4. A process as claimed in claim 1, wherein the reaction gasstarting mixture comprises ≧60% by volume of propane.
 5. A process asclaimed in claim 1, wherein the reaction gas starting mixture comprises≧70% by volume of propane.
 6. A process as claimed in claim 1, whereinthe reaction gas starting mixture comprises ≧20% by volume of O₂.
 7. Aprocess as claimed in claim 1 which is carried out continuously.
 8. Aprocess as claimed in claim 1, wherein the molar ratio of propane tomolecular oxygen in the reaction gas starting mixture is <5.
 9. Aprocess as claimed in claim 1, wherein the ratio of the bed volumes ofthe two catalyst beds A, B is from 1:5 to 5:1.
 10. A process as claimedin claim 1, wherein the reaction pressure is >1 bar.
 11. The process ofclaim 1, wherein M¹ is Co, Ni, and/or Mg and M² is Sn, W, P, Sb, and/orCr.
 12. The process of claim 1, wherein M¹ is Co and/or Ni and M² is W,Sn, and/or Sb.
 13. The process of claim 1, wherein X¹ is W, X² is Co,Ni, and/or Zn, X³ is Fe, X⁴ is P and/or Sb, X⁵ is K and/or Na.
 14. Theprocess of claim 1, wherein X⁶ is Si, Zr, Al, Nb, Ag, and/or Ti.
 15. Theprocess of claim 1, wherein X⁶ is Si.
 16. The process of claim 1,wherein e′ is 0.5-10.
 17. The process of claim 1, wherein X⁶ is Si ande′ is 0.5-10.